Hair grows about an inch every couple of months and each hair sits in a cavity called a follicle. Hair typically grows for 2 to 6 years, remains at the final length for a short period, and then falls out. A new hair soon begins growing in its place. At any one time, about 85% of the hair on your head is in a growing phase and 15% is not.
Hair grows in naturally occurring bundles of one to four follicles called follicular units. Permanent hair loss occurs in men when follicles containing genetic sensitivity to dihydrotestosterone or DHT are triggered to atrophy or shrink over time, resulting in shorter and finer hair.
The end result is a very small follicle with no hair inside. Ordinarily, hair should grow back. However, in men who are balding, the follicle fails to grow a new hair. This genetic hair loss typically presents itself in a defined pattern called Male Pattern Hair Loss and can begin anytime after puberty.